Certification should require a demonstration of knowledge and understanding, which is why all Scrum.org programs employ rigorous assessments. In further pursuit of that goal, Scrum.org is now making passing it's Professional Scrum Master I (PSM I) Assessment a prerequisite for the PSM II Assessment to ensure students are well prepared for the more advanced evaluation.
Previously, PSM II Assessments were readily available to anyone: those who have a fundamental knowledge of Scrum, and in some cases, those that don't. The more fundamental PSM I examination has proven an important step in succeeding with PSM II, as voiced by Scrum.org trainers and students.
A more defined path for demonstrating Scrum acumen helps Scrum.org recognize those who have truly demonstrated understanding and continues to assure the strength of the PSM II certification.
You can read more about the Professional Scrum Master program here.